One of the
largest hurricanes to make landfall in the U.S. Gulf Coast region since Hurricane
Camille in 1969, Hurricane Katrina left a trail of devastation behind it as it
touched down in Louisiana, Mississippi and Alabama on Aug. 29. The storm affected
not only the Gulf communities and shorelines it passed, but also the oil and gas
market (see story, page 48, print
exclusive).
The eye of Hurricane Katrina, shown here in an image taken by NASAs GOES
satellite, sat just east of New Orleans on the morning of Aug. 29, before moving
north into Mississippi. Courtesy of NASA.

After the storm deluged Florida, it moved west, intensifying from a Category-2
storm to a Category 5 over the Gulf, according to the National Oceanic and Atmospheric
Administrations Hurricane Warning Center, before diminishing to Category
4. Thousands of people had been evacuated from New Orleans, which lies below sea
level owing to compaction and subsidence in the region.

The eye moved farther east than originally predicted, which misled residents in
the region into thinking the worst was over. But after the storm passed, floodwaters
breached the floodwalls holding back Lake Pontchartrain and the 7th Street Canal,
drowning New Orleans. The entire city was evacuated in the following days, and
on Aug. 31, the mayor said that thousands of people may have died, although the
death toll is still uncertain. Most residents had yet to return as of Sept. 15.

The condition of the levees has been a concern over the past decade, says Peter
Scholle, president of the Association of American State Geologists and New Mexico
State Geologist. The body of literature published on the Mississippi basin and
other discussions on the state of the levees have led to a lot of public
praying about New Orleans, he says, in the event of a large storm or flood.
The whole system has been a faith-based approach to coastal management.

Although Hurricane Katrina is one of the largest storms on the record, it is not
out of the ordinary. Such extreme events are part of a multi-decadal cycle (see
Geotimes Web Extra, Aug. 8, 2005); the last surge
in such storms occurred in the 1950s and 1960s. Climate change most likely has
little to do with such shifts in hurricane patterns.

A
typical thunderstorm often illustrates that lightning and rain go together 
but not always. The more important partnership may be between lightning and
ice in a storm cloud. New observations recently confirmed that lightning follows
clouds ice content, potentially providing climate scientists with a new
method of measuring water in the atmosphere worldwide  an important component
of global climate models for forecasts.
This satellite image of Hurricane Katrina, created by combining radar and radiometer
data together from the moment when the storm made landfall, shows the hurricanes
increased precipitation as it grew to a Category-5 storm. Little white spikes
mark where the Tropical Rainfall Measuring Missions Lightning Imaging
Sensor detected flashes of lightning. Note that the detected lightning occurs
in the outer spiral bands of the hurricane, and not at the center. That difference
may be because the clouds forming Katrinas eye lacked precipitation-sized
ice, a conclusion that requires further research. In the meantime, a new study
has shed light on the role of ice in the occurrence of lightning. Image by Dennis
Boccippio, NASA Marshall Space Flight Center.

It rains hard usually when you have lightning, says Walter Petersen
of the Earth System Science Center at the University of Alabama in Huntsville,
but lightning can also happen without rain. However, lightning always occurs
in the presence of ice particles, something determined with observations and
lab experiments in the 1940s and 1950s. The experiments, Petersen says, showed
how three water phases present in a cloud could make graupel pellets 
slightly smaller than hail but bigger than ice crystals  transfer an electrical
charge.

The more graupel, the more lightning, Petersen says, likening a thunderstorm
cloud to a battery. As water vapor is blown up into the cloud, it cools and
condenses to form small cloud droplets and then larger raindrops. These bits
of water become supercooled and eventually freeze as they move up, accreting
together with other ice particles to make graupel pellets, which tend to fall
back down. At the same time, thunderstorm updrafts move ice crystals up in the
cloud. As passing ice crystals collide with the falling graupel, they exchange
electrons. The movement creates an electrical current, basically making one
end of the cloud positive and the other negative  perfect for making lightning.

With that knowledge in hand, Petersen and colleagues examined satellite data
from NASAs Tropical Rainfall Measuring Mission (TRMM) to look at lightning
flashes around the world, along with the locations of water and large ice particles.
The team compared ice density in the clouds with the amount of flashes produced,
and found that the correlation of the amount of ice with the amount of lightning
stayed steady, whether over land masses or oceans: Increasing the amount of
ice increased lightning flashes proportionally.

If you look at the oceans or the continents or coastal areas, all these
different areas have different ways to make rainfall, Petersen says. The
thing that is consistent in those areas is the mechanism to make lightning:
the presence of this ice mass.

The new research, published in Geophysical Research Letters on July 26,
is excellent and probably the best documented if not
the first confirmation of the steady correlation between lightning and precipitation-sized
ice in thunderstorm clouds, says Ken Pickering of the University of Maryland
in College Park.

Because ice is part of the water budget, Petersen says, it is an integral piece
of information for weather and climate modeling. But just how much ice contributes
to the system remains unknown. The research team suggests that lightning measurements
could be a good indication for how much water-ice is present in clouds on a
global scale.

Pickering also suggests that the findings could also help in determining the
chemical impact of lightning on the atmosphere, which catalyzes the production
of ozone by creating nitric oxide  a possible climate feedback loop. Current
satellite instruments, he says, cant get an exact count of all lightning
flashes around the world, and chemical modelers must make estimates based
on meteorological variables. We may be on a better track to predicting
global flash rates better, Pickering says, for the next generation of
climate models.